Siva advises Markandeya Rishi.

Markandeya was the son of Mirukandu Rishi who as soon as his son was born realised that his son would live only for sixteen years. so when the boy came of age he advised the boy Markandeya to offer sincere prayers in order to get over the impeding calamity, he told him to go to Palasa vanam and pray to Mahadeva at Appakudataan kshetram. Markandeya did as his father requested him after having a bath in the Indra puskarini. The lord one day appeared before him as a swan and started fanning him with his feathers and creating some commotion in the water. the Rishi first agitated tried to dissuade the swan saying he could not help him he said even Indra cannot help me etc. The swan calmly said he was a special bird he was the king among the swans and he belonged to the Saithara ratham which was under the administration of Kubera himself and that he lived in the Manatham tank known to be golden in colour filled with flowers. he asked the boy Markandeya to share his worries as he was well aware of all the three worlds and that he would try to solve the boys problem. the boy being hesitant he finally said he could even save him from the clutches of Yama. On hearing this Markandeya thought that his prayers were answered. He told the swan that he was there on his fathers request and that he was praying to Lord Siva and as the swan had said that he would save him from Yama too he felt his prayers to the Lord were answered. Siva was pleased with this and appeared in his real form and advised the boy to pray to Lord Narayana. he taught him one mantra which he said was very effective and which would act as a kavacham even against Yama kinkaras.
it goes like this:

Kesava please guard my head;
Narayana please guard my Hair;
Madhava please guard my forehead;
Govinda please guard my eyes;
Vishnu, Madhusudhana please guard my nostrills and the ears;
Thrivikrama plase guard my neck;
Vamana and Sridhara please guard my two lips;
Hrishikesa please guard my teeth;
Padmanabha please guard my tongue;
Damodara please guard my two shoulders;
Shankarshana please guard my chest;
Vasudeva please guard my stomach;
Pradhyumna please guard my back;
Aniruddha please guard the two sides;
Purushothama please guard my waist;
Narasimha please guard the two thighs
Achutha please guard my two knees;
Janardhana please guard the kanukkal, anklets;
Upendra please guard my feet;
Hari please guard the toes;
Krishna please guard my entire body.
As advised by Lord Siva the boy Markandeya used to have his daily bath in the Puskarini and pray with the above kavacham to Lord Narayana. 
Yama doothas when they approached him were frightened as he was taking Keshava's name and rushed off to Yama to complain.
On seeing Yama himself come Markandeya prayed to Shiva again and was once again advised to approach an old Brahmin who was sleeping in the house of the King Uparisaravasu {Another story of the old brahmin being here) as he was Lord Narayana himself.
Markandeya did so and was blessed by the old Brahmin who was holding in one hand appakudam lovingly.  the Nitya puskarini thenceforth came to be known as Mruthyu vinashini.

The story of the old Brahmin with the appakudam.
A Pandya raja by name Uparisaravasu was advised by Siva to pray to Lord Narayana with a prayer which he said was taught by Brahma to Indra. he was advised to repeat the sloka 1008 times and offer food to brahmins with appam ghee and payasam daily. The King religiously followed this advice and continue to stay there.
One day Lord Narayana took the guise of an old Brahmin and approached the King and told him that he was hungry and wanted to be fed immediately. the king reluctantly did so the old Brahmin ate up all the food prepared for the100's of brahmins and looked still hungry. the King requested the Brahmin to take some rest while he made arrangements to prepare some more food. the King then prepared appams with lots of ghee and bhakti to feed the old man. He had bought the appam's in a Kudam. the Lord stretched his hands and took the Kudam and gave the King his blessings of long life and moksha.