I introduced this ability to the magic-user class as a house rule in my AD&D 1e game, in order to give more flavor to the bond between the magic-user and his familiar.
Familiar Link: The magic-user, if he concentrates as when casting a spell, has a 10%/level ability to establish a telepathic connection with the familiar. This percentage is modified by the distance to the familiar (-2% per mile) and the maximum distance to establish the connection is equal to the range of the Find Familiar spell (1 mile/level). The connection will last as long as the magic-user is focused on maintaining it. Through that connection he will be able to feel the same as the familiar feels, and also send simple commands, as long as they do not go against the familiar's survival instinct.
