مُسْنَدُ أَحْمَدَ
Musnad Ahmad 3721
Musnad of Abdullah bin Mas'ood
It was narrated that ‘Abdullah (رضي الله عنه) said - Abu Ahmad said: that Ibn Mas'ood said - The Prophet (ﷺ) liked to repeat supplications three times and prayers for forgiveness three times.
Musnad Ahmad 3744its isnad is saheel]