The Mitteler Rebbe (who lived in the same house as his father, the Alter Rebbe) was once sitting in his room deeply engrossed in Torah study while, in the same room, his youngest child slept quietly in a crib. Suddenly, the baby rolled out of his crib onto the floor and began crying loudly. So immersed in contemplation was the Mitteler Rebbe that he did not hear anything and he continued his studies.

The Alter Rebbe was sitting in his room, farther away from the baby's room. Although he was also immersed in study, he nonetheless heard the baby crying. He entered the room and put the baby back in the crib. After he had rocked the baby back to sleep, he returned to his room. A while later, the Alter Rebbe rebuked his son and said: "One must never be so immersed in his studies that he does not hear the cry of a child."

Toras Menachem 5714, vol. 1, p. 229ff.