Mishnah 22 - Chapter 5 - Pirkei Avot

Ben Bag Bag says: Turn it [Torah] over and over, for [you will find] all in it. And through it you shall see. And grow old and hoary in it, and do not budge from it, for there is no better middah than it. [Do not say: I have learned the wisdom of Israel; I will now go and learn the wisdom of the Greeks. For it is permitted to do the latter only where it is not permitted to meditate in words of Torah, as in the bath-house or the lavatory. When they asked R. Yehoshua whether it was permitted to teach one's son Greek wisdom, he answered that it was permitted to do so when it was neither day nor night, it being written (Joshua 1:8): "And you shall meditate in it day and night."]