مُسْنَدُ أَحْمَدَ
Musnad Ahmad 2172
Musnad of 'Abdullah bin al-'Abbas bin 'Abdul-Muttalib from the Prophet
It was narrated that Ibn 'Abbas (رضي الله عنه) said: Allah, may He be glorified and exalted, enjoined prayer on the lips of your Prophet (ﷺ): when not travelling, four rak'ahs; when travelling. two rak'ahs; and at times of fear, one rak'ah.
Musnad Ahmad 2177Its isnad is saheeh]