Musannaf Ibn Abi Shayba 3:5843

Waki’ narrated to us, he said: Qurrah ibn Khalid narrated to us, from Abu Hakimah, he said: I asked Sa’id ibn al-Musayyib, and he said: "If you intend to stay for three [days], then complete the prayer." Waki’ said: I heard Sufyan saying: "If he resolves to stay for fifteen [days], he completes the prayer as soon as he enters. And if he does not know when he will leave, he prays two rak'ahs, even if he stays for a year." And that is the opinion held by him.