27 It is also well known that our Saint, desiring to call back men so that they would conform their lives to the teachings of the Gospel, used to exhort them "to love and fear God and to do penance for their sins." ( Legend of the Three Companions , No. 33 et seq) Moreover, he preached and invited all to penance by his own example. He wore a hair shirt, he was clothed in a poor rough tunic, went about barefoot, he slept resting his head on a stone or on the trunk of a tree, ate so little that it was barely sufficient to keep him from dying of starvation. He even mixed ashes and water with his food in order to destroy its taste. He passed the greater part of the year in fasting. Besides all this, no matter whether he was well or ill, he treated his body with the greatest severity; he used to call his body "my brother the ass"; nor could he be induced to give himself any relief or rest, not even when, as during the last years of his life, he was suffering greatly, the sufferings of one nailed to a cross, for he had become like unto Christ because of the stigmata which he bore. Neither did he neglect to inculcate austerity of life in his disciples, and, in this only did "the teachings of the Holy Patriarch differ from his own actions," (Thomas of Celano, Legenda II, No. 129) he advised them to moderate a too excessive abstinence or punishment of the body.
Source: Rite Expiatis (Vatican.va)