"Rabbi Meir used to say: He that studies Torah with a single teacher, to whom may he be likened? To one who had a single field, part of which he sowed with wheat and part with barley, and planted part with olives and part with oak trees. Now that man is full of good and blessing. But when one studies with two or three teachers he is like him who has many fields: one he sows with wheat and one he sows with barley, and plants one with olives and one with oak trees. Now this man's (attention) is divided among many pieces of land, without good or blessing."
Avos d'Rabbi Nasan Chapter 8