[Q&A] Where does the believe "monks mother will be reborn in heaveb" come from?

Once answered:

In the case a mother gives her most beloved, let it go forth, that being a very high generosity , it is out of this reason, that she gains heavenly existence. Generosity (letting go of objects of the five senses, matter) is the cause of gaining heavenly existence (sugati realms).

In the case a mother rejoices this the gain of higher for her relative, mudita , such non-stinginess and joy of gains of others, is the case of gaining existence in the Brahma realms.

In the case a mother can at least mentally follow the path of leaving home into higher existence and happiness, such being the cause, seeing even the Dangers in the world, is the cause to gain existence in the Arupa-heavens.

If a mother follows the son's way of practice, goes for refuge into the Tripple Gems as well, such as the parents of Yasa, meeting the Buddha and his teachings, by the fact of entering the stream, on hearing it, she is destination to be no more able to fall of and as most Sotapannas, will gain heavenly realms.

Mv I 07: Pabbajjākathā — The Discussion of Going-forth [Ven. Khematto]. Yassa, a son of a money lender, disgusted of sense pleasure met the Buddha in the forest. After having been arisen to the highest Dhamma, his parents, on search for him, became, with his former wife, the first three-fold lay followers.

A very famous and inspiring story, next to that of the Buddhas Mother, Mahāmāyā , is the story of the mother of Ven. Sariputta Maha Thera, late then Upasika, Rūpasārī .

...Both she and her husband were unbelievers, and she was very sad when, one after another, her children, giving up wealth worth eighty crores, joined the Order. She wished to keep at least the youngest of the boys, Revata, for herself, and had him married at the age of seven, but her plot miscarried (See Revata). This embittered her against the monks, and, though she gave them alms when they came to the house, she blamed them for having enticed her children away...

This all being the reason, being the cause, having gained relation the family of the Noble Ones, is why it is said that the monk's mother (as she has to agree that one could become) is destined to heaven.

A very good topic, good to investigate.

Anumodana