Al-Matalib al-'Aliyah 595 — Tahajjud

It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) raised his blessed gaze towards the sky, then lowered it, so we said: "O Messenger of Allah! Why did you do this?" He (peace and blessings be upon him) said: "I was amazed at two angels from among the angels who came down to the earth to look for a (righteous) servant at his place of prayer, but they did not find him there. Then the two of them ascended to their Lord and submitted: 'O our Lord! We used to write for this believing servant of Yours such and such good deeds in his day and his night, but (this time) we found that You had confined him in Your grip (of illness and trial), so we did not write anything in his record of deeds.' Then Allah, the Blessed and Exalted, said: 'Write for My servant all the deeds of his day and his night that he used to do before, and do not reduce anything from them at all. His reward is upon Me, because it is I who have held him back (through illness), so he will have the reward of all the deeds he used to do continually (in the state of health).'"
Musnad Ahmad: 3821, Musnad Abi Ya'la: 5122, Majma' al-Zawa'id: 2/303. Allamah al-Albani (may Allah have mercy on him) declared it sahih (authentic) in 'Silsilat al-Ahadith al-Sahihah' (2092).