Humayd bin Qays al-Makki (may Allah have mercy on him) said: I was performing tawaf with Mujahid (may Allah have mercy on him), when a man came and asked: Is it necessary to keep the fasts of expiation consecutively? Humayd (may Allah have mercy on him) said: I said: No. Mujahid (may Allah have mercy on him) struck my chest and said: In the recitation of Ubayy (may Allah be pleased with him) there are the words "mutatabi'at" (consecutive), meaning he should keep them consecutively.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الأيمان / 20009