In Hac Tanta §17

¹⁷ Moved by a very strong faith and burning with love and piety, Boniface seems to have drawn his unique and faithful union to the Holy See first from the contemplative life of monasticism in his own country. Later, when he was about to undertake the difficulties of the apostolic life, he promised this fidelity at Rome by an oath at the tomb of Saint Peter, prince of the apostles. He exhibited this fidelity in the midst of dangers and struggles as the mark of his apostleship and the rule of his mission. He never relented from recommending this fidelity to all those for whom he was a father in Christ. In fact, he was so diligent that it seemed he desired to leave it to them as an inheritance.