مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3746
Musnad of Abdullah bin Mas'ood
It was narrated that 'Abdullah (رضي الله عنه) said: The Messenger of Allah (ﷺ) liked to repeat supplications three times, and prayers for forgiveness three times.
Musnad Ahmad 3769Its isnad is saheeh]