Fiqh-us-Sunnah Volume 2, Supererogatory Prayer, Fiqh 2.002A.

It is preferred to prolong the reciting by making many rak’at. The group, except for Abu Dawud, reports that al-Mughirah ibn Shu’bah said: “The Prophet sallallahu alehi wasallam would stand and pray until his feet or shanks swelled. When he was asked about it, he said: ‘Should I not be a thankful slave?”‘ Abu Dawud records from ‘Abdullah ibn Hubshi al-Khath’ami that the Prophet sallallahu alehi wasallam was asked: “What is the best deed?” He said: “Prolonging the qiyam; (standing) [in the prayer].” Then it was asked: “What is the best charity?” He replied: “The sacrifice made by one who has little to give.” Then it was asked: “What is the best migration?” He responded: “The migration from what Allah has forbidden.” Then it was asked: “What is the best jihad?” He replied: “Whoever strives against the polytheists with his wealth and soul.” They asked: “What is the most honourable death?” He answered: “He whose blood is spilled and whose horse is wounded.”