I asked Anas b. Malik about shortening of prayer. He said: When the Messenger of' Allah (ﷺ) had covered a distance of three miles or three farsakh (Shu'ba, one of the narrators, had some doubt about it) he observed two rak'ahs.
Reference: Sahih Muslim 691 In-book : Book 6, Hadith 14 USC-MSA web (English) : Book 4, Hadith 1471 (deprecated)
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