Lucius Annaeus Seneca is our author.
Please tell me, which of his writings contains the phrase:
Which of his books/letters contains the quote?
Wiki sez De Beneficiis. Off to check chapter and verse…
deleted
Yay the Perseus Project! 
De Beneficiis book 4, chapter 26, section 1:
That is, more or less, quoting some source I haven’t identified:
It is said, “If you want to imitate the gods, give benefits to the ungrateful too; for the sun shines also on the wicked, and the seas lie open to pirates.”
Book IV, Chapter I page XXVI of On Benefits
This man uses a lot of solar imagery in his writing.
You’re welcome. 
My apologies for not thanking you all sooner.
Must’ve been a popular expression:
43 “You have heard that it was said, ‘You shall love your neighbor and hate your enemy.’ 44 But I say to you, Love your enemies and pray for those who persecute you, 45 so that you may be children of your Father in heaven; for he makes his sun rise on the evil and on the good, and sends rain on the righteous and on the unrighteous. 46 For if you love those who love you, what reward do you have? Do not even the tax collectors do the same? 47 And if you greet only your brothers and sisters,[o] what more are you doing than others? Do not even the Gentiles do the same? 48 Be perfect, therefore, as your heavenly Father is perfect. -Mathew, Chapter 5 (The Sermon on the Mount)
Probably the “Where’s the beef?” of the Roman Empire.