Ibn Abbas (may Allah be pleased with them both) was amazed at those people who begin the month in advance, even if the moon has not been sighted, and he used to say that the Prophet (peace and blessings be upon him) said: "If the moon is not sighted, then complete the count to thirty."
Hadith Referenceمسند احمد / ومن مسند بني هاشم / 3474
Hadith Gradingحکم دارالسلام:إسناده ضعيف، محمد بن جبير وهو خطأ، صوابه محمد بن حنين وهو مجهول