It is narrated from Al-Husayn ibn Sa’id, from Al-Nadr, from Ibn Sinan, who said:
I heard Abu Abdullah عليه السلام say: "The woman in a state of nifas (postnatal bleeding) refrains from prayer for nineteen nights. If she still sees blood after this, she follows the same rulings as the mustahadah (a woman experiencing irregular bleeding)."
Ali Akbar al-Ghaffari
Tahdhīb al-Aḥkām fī Sharḥ al-Muqniʿah, vol. 1, p. 187