Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) narrated that the Messenger of Allah (peace and blessings be upon him) said: "Indeed, Allah Almighty has lengthened it in order to show it to you; if it is clouded over you, then complete the count."
Ikrimah narrated from Sayyiduna Abdullah bin Abbas (may Allah be pleased with them both) concerning completing the count of thirty days, and others besides him narrated it as well.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7936