Ahmad said: The name Ibn al-Thalabb is to be pronounced with a ta' (and not with tha). As Shu'bah could not pronounce ta, he said tha.
In-book : Book 31, Hadith 23
English translation : Book 30, Hadith 3937
31. The Book of Manumission of Slaves
باب فِيمَنْ رَوَى أَنَّهُ، لاَ يَسْتَسْعِي