Muwatta Malik, Book 18, Hadith 45 (USC-MSA) ⓘ
Yahya related to me from Malik from Nafi that Abdullah ibn Umar used to say, "Someone who breaks the fast in Ramadan because he is ill or travelling should make up the days he has missed consecutively."
وَحَدَّثَنِي عَنْ مَالِكٍ، عَنْ نَافِعٍ، أَنَّ عَبْدَ اللَّهِ بْنَ عُمَرَ، كَانَ يَقُولُ يَصُومُ قَضَاءَ رَمَضَانَ مُتَتَابِعًا مَنْ أَفْطَرَهُ مِنْ مَرَضٍ أَوْ فِي سَفَرٍ .
also referenced under: Book 18, Hadith 678 (In-Book)