Muwatta Malik, Book 16, Hadith 538 (In-Book)

Yahya related to me that Malik asked Ibn Shihab about a man who caught some of the takbirs said over the corpse and missed the rest, and Ibn Shihab said, "He completes what he has missed."

وَحَدَّثَنِي عَنْ مَالِكٍ، أَنَّهُ سَأَلَ ابْنَ شِهَابٍ عَنِ الرَّجُلِ، يُدْرِكُ بَعْضَ التَّكْبِيرِ عَلَى الْجَنَازَةِ وَيَفُوتُهُ بَعْضُهُ فَقَالَ يَقْضِي مَا فَاتَهُ مِنْ ذَلِكَ ‏.‏

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also referenced under: Book 16, Hadith 16 (USC-MSA)