Answer:
The source of income is the capital gain realized when Harry sold the stocks of Extel Corporation. Generally, nonresident aliens (like Harry) are subject to a 30% tax on all their US income sources. E.g. if Harry made a capital gain of $1,000 when he sold the stocks, he will need to pay $300 to the IRS.
Some exemptions apply to foreign students, resident aliens or people that work for foreign governments, but Harry doesn't fit in any of these categories.