Abu Salamah (may Allah have mercy on him) narrated from Hammam that Qatadah (may Allah have mercy on him) said: They differed about a day of which it was not known whether it was from Ramadan or from Sha'ban, so we came to Anas (may Allah be pleased with him) and found him sitting and eating his morning meal. Hudhayfah bin al-Yaman (may Allah be pleased with him) used to forbid fasting on the day of doubt, and Ibn Abbas (may Allah be pleased with them both) used to say: Make a separation between Sha'ban and Ramadan by not fasting.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصيام / 7960