Sahih Muslim Book 6, Hadith 66
Ibn 'Abbas reported that the Messenger of Allah (ﷺ) observed in Medina seven (rak'ahs) and eight (rak'ahs), i. e. (be combined) the noon and afternoon prayers (eight rak'ahs) and the sunset and 'Isha' prayers (seven rak'ahs).
I observed with the Messenger of Allah (ﷺ) eight (rak'ahs) in combination, and seven rak'ahs in combination. I (one of the narrators) said: O Abd Sha'tha', I think that he (the Holy Prophet) had delayed the noon prayer and hastened the afternoon prayer, and he delayed the sunset prayer and hastened the 'Isha' prayer. He said: I also think so.
Ibn 'Abbas one day addressed us in the afternoon (after the afternoon prayer) till the sun disappeared and the stars appeared, and the people began to say: Prayer, prayer. A person from Banu Tamim came there. He neither slackened nor turned away, but (continued crying): Prayer, prayer. Ibn 'Abbas said: May you be deprived of your mother, do you teach me Sunnah? And then he said: I saw the Messenger of Allah (ﷺ) combining the noon and afternoon prayers and the sunset and 'Isha' prayers. 'Abdullah b. Shaqiq said: Some doubt was created in my mind about it. So I came to Abu Huraira and asked him (about it) and he testified his assertion.