Can you please tell me what page or what chapter Dr. Mortimer is trustworthy?

Keeping his word.... from the text:

“The exact date is 1742.” Dr. Mortimer drew it from his breast-pocket. “This family paper was committed to my care by Sir Charles Baskerville, whose sudden and tragic death some three months ago created so much excitement in Devonshire. I
may say that I was his personal friend as well as his medical attendant. He was a strong-minded man, sir, shrewd, practical, and as unimaginative as I am myself. Yet he took this document very seriously, and his mind was prepared for just such an end as did eventually overtake him.”