A passage from Thomas Aquinas on labor by hand (which, of course, is what 'manual labor' means):
Manual labor is directed to four things.
First and principally to obtain food; wherefore it was said to the first man (Genesis 3:19): "In the sweat of thy face shalt thou eat bread," and it is written (Psalm 127:2): "For thou shalt eat the labors of thy hands."
Secondly, it is directed to the removal of idleness whence arise many evils; hence it is written (Sirach 33:28,29): "Send" thy slave "to work, that he be not idle, for idleness hath taught much evil."
Thirdly, it is directed to the curbing of concupiscence, inasmuch as it is a means of afflicting the body; hence it is written (2 Corinthians 6:5-6): "In labors, in watchings, in fastings, in chastity."
Fourthly, it is directed to almsgiving, wherefore it is written (Ephesians 4:28): "He that stole, let him now steal no more; but rather let him labor, working with his hands the thing which is good, that he may have something to give to him that suffereth need." [ST 2-2.187.3]
(It's rather notable that three of the four ends of manual labor have to do with penitential practice and discipline for virtue. In context, Aquinas is walking a very fine line -- manual labor is a requirement of natural law, but manual labor was a controversial theological topic in his day, because monks were required to do manual labor to support themselves, while the mendicant orders, like the Dominicans to whom Aquinas belonged, did not have such a requirement, and this was often seen as a reason for rejecting this strange innovation of monks without monasteries. So Aquinas also wants to argue that one may reasonably and morally devote oneself to higher pursuits than manual labor -- it does belong to natural law that the human race must support itself by manual labor, but if you, as an individual, can support yourself some other way by an even more noble form of labor (like teaching), in circumstances in which manual labor is not a necessity for survival, then you do not need to do manual labor except, perhaps, as an occasional penitential practice.)