A person is praying Fajr alone. After completing the first Sajdah, the Fajr congregation is established. What does this source say to do?

When Congregation Begins during an Individual Fard or Sunnah Prayer

• End the individual prayer with one Salaam and join the congregation.
• Always complete the individual Fajr first.
• Convert the prayer into Witr.
• Remain sitting until congregation ends.