…do not harden your heart or close your hand… (Deuteronomy 15:7)
There is a prohibition against refraining from giving charity to those in need, especially when it comes to those close to us, to whose financial straits we are privy. We are not to let the greedier side of our natures get the better of us and talk us out of doing the right thing.
The reason for this is the same as the reason generally underlying the obligation to give charity: God appoints us as His agents to help support one another. If we perform acts of kindness for one another, we become more deserving of receiving kindness from Him.
This mitzvah applies in all times and places. It is discussed in the Talmud in tractate Baba Basra (9a-10b) and in Kesubos (50a). It is codified in the Shulchan Aruch in Yoreh Deah 247. This mitzvah is #232 of the 365 negative mitzvos in the Rambam’s Sefer HaMitzvos and #62 of the 194 negative mitzvos that can be observed today as listed in the Sefer HaMitzvos HaKatzar of the Chofetz Chaim.