He says this during the obligatory prayer. (1)

84 - Chapter: Another [Hadith] on the Same Topic

266 - Al-Ansari narrated to us, saying: Ma'n narrated to us, saying: Malik narrated to us, from Sumayy, from Abu Salih, from Abu Hurayrah, that the Messenger of Allah, peace and blessings be upon him, said: "When the Imam says: 'Sami'a Allahu liman hamidah' (Allah hears the one who praises Him), then say: 'Rabbana wa laka al-hamd' (Our Lord, and to You be all praise). For whoever's saying coincides with the saying of the angels, his previous sins will be forgiven." (2)

This hadith is hasan sahih.

The practice according to some scholars among the Companions of the Prophet, peace and blessings be upon him, and those after them, is that the Imam says: "Sami'a Allahu liman hamidah," and those behind the Imam say: "Rabbana wa laka al-hamd." This is the view of Ahmad.

Ibn Sirin and others said: Those behind the Imam should say: "Sami'a Allahu liman hamidah, Rabbana wa laka al-hamd," just as the Imam says. This is the view of al-Shafi'i and Ishaq.

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(1) After this, in the edition of Shaykh Shakir, the following appears: Abu 'Isa said: He is only called "al-Majishun" because he is a descendant of al-Majishun, and this is not found in any of our manuscript sources.
(2) Sahih. Recorded by al-Bukhari (796), Muslim (409), Abu Dawud (848), and al-Nasa'i 2/196. It is also in al-Musnad (9923).