Musnad Aḥmad 3/98, no. 11946. Its isnād is ṣaḥīḥ according to the conditions of al-Bukhārī and Muslim.
Ghusl is the full washing of the whole body that is obligatory after sexual intercourse or any discharge of semen, whether while awake or in a wet dream. A man who has intercourse more than once, whether with the same wife or more than one lawful wife, incurs one state of janābah; one complete ghusl after he is finished validly removes it. He does not owe a separate obligatory ghusl after each act. This is the position of the jurists generally, and the Anas report is direct evidence for it. There is, however, a more complete practice between the two acts: Abū Saʿīd al-Khudrī narrated that the Prophet ﷺ said: “If one of you has intercourse with his wife and then wishes to return, let him perform wuḍūʾ between the two [acts]” (Ṣaḥīḥ Muslim, no. 308). The majority understand this wuḍūʾ as recommended, not obligatory. There is also the report of Abū Rāfiʿ that the Prophet ﷺ once washed after each wife, saying, “This is purer, cleaner, and better” (Sunan Abī Dāwūd, no. 219; Sunan Ibn Mājah, no. 590; Musnad Aḥmad, no. 23862). Its chain is hasan / capable of strengthening; Abū Dāwūd himself noted that Anas’s report is stronger. The two practices are not contradictory: one ghusl is permissible and sufficient, while washing or making wuḍūʾ between acts is cleaner and more virtuous.