In the US1, it is commonly said that rain on your wedding day is good luck—of course, it's also said that "happy is the bride who the sun shines on". In my experience, you can pick whichever superstition fits the weather at the wedding you are currently attending.
Several wedding sites note that "a wet knot is harder to untie", which might suit your wish for a pithy saying, but I don't know how English it is, or how well-known it is outside of wedding planning circles—the claim in those circles seems to be that it is of "Hindu" origin2, and it's not something I'd heard before looking for a source for the general superstition.
1 I can't speak from personal experience, but at least one UK magazine also says:
We're not sure any couple really wants torrential rain on their wedding day, but many cultures, including us in Britain, have long associated a bit of drizzle with good luck.
—Francesca Rice, "Funny wedding superstitions and traditions explained", Prima, 5 January 2017
2 The first example I can actually find in print for this is from a 2004 newspaper story about local (Coronado, California) couples. The husband in one of the featured couples recalls of their 1984 wedding:
“We had a great wedding, but it rained. A lot. It was torrential rain for San Diego. A friend gave me a phrase for our marriage that day. 'A wet knot is hard to untie.’ I’ve remembered that ever since then.”
—Rebecca Waer, "There are love stories to be found in Coronado", Coronado Eagle and Journal, 11 February 2004, p. 28
There is no mention of the friend's religion or national origin. The next year, a wedding planning book also mentions the saying, the first example I can find in Google Books:
Since wedding lore from various cultures is contradictory as to whether rain is a good or bad omen on your wedding day—think positive! As one encouraging old saying goes, “A wet knot is more difficult to untie,” so a long married life may await those who marry under rain clouds.
—Kim Knox Beckius, The Everything Outdoor Wedding Book, 2005
Despite the mention of "various cultures", there is no origin given for this "old saying". The Hindu derivation is given for the first time that I can find in a 2009 memoir (also the second Google Books result, chronologically):
According to Hindu tradition, rain on your wedding day is a sign of good luck as it signifies a strong marriage. The premise behind this belief is that a wet knot is harder to untie.
—Kathleen Lockwood, Major League Bride, 2009
A 2009 "wedding planning blog" also repeats this story:
Research suggests that the superstitious sane[sic], “Rain on your wedding day is good luck” comes from several cultures mainly from the Hindu, with the idea that since marriage is often referred to as tying the knot, a wet knot is harder to untie.
—Claudia Lutman, "A Look Back on Shejal & Josh, Sept. 6, 2008", Strive to Be Gracious – A Wedding Planning Blog, 6 September 2009
Although the bride in this entry was apparently Hindu (judging from the description of the ceremony by the wedding planner), it's unclear what "research" the writer is actually referencing.
After that there are several more mentions of the tradition in print between 2010 and 2016, some with the (unsourced) "Hindu" origin story, some without, and there are now numerous repetitions of the quote on various wedding-related websites. Based on this evidence, it seems plausible to me that it is a saying from the Indian sub-continent, but also that it was originally coined by an English-speaking officiant or friend seeking to comfort a rained-on bridal couple.